A semiconductor apparatus having a region for isolation between devices comprises a semiconductor substrate, a polycrystalline silicon layer portions selectively formed to be spaced apart from each other on the semiconductor substrate, an impurity diffused region formed under the polycrystalline silicon layer, and a silicon oxide film for filling in a space between the respective adjacent portions of the polycrystalline silicon layer. The impurity diffused region constitutes a source or drain region of a field effect device such as a MOS transistor isolated by the silicon oxide film.